Handle dynamic bodies without mass better, so user cant easily make mistake

[viblo]

As clear from #267 and #226 and others its really easy to forget to set a valid mass on a body. And when it happens you will get a difficult-to-understand error, most common is debug draw failure because of NaNs. This must be possible to handle better.

[viblo]

The problem with this error is that is not easy where to stop the 0 mass.

• Its valid to create a body with 0 mass, if you attach a shape with mass to it before simulation step.
• Chipmunk contains already an assert that checks that if a shape attached to a body has mass >0, then the body have a valid mass calculated.
• I dont want to have to check the mass every call to step. I do however think its reasonable to require that bodies have >0 mass once the simulation is run even if that is not the case now.
• Even if debug draw had a better error message, there's no guarantee that debug draw is used, so does not solve it fully.
• Bodies are not fully connected to the shapes before they are added to a space.

To be able to check for this the API needs to be more strict.

The least breaking solution I can think of is:
Restrict so that if you have a body with 0 mass, add a shape to it and add the shape to the space, at this time the body must get a mass > 0. This means you cant do a 0 mass body, add 0 mass shape to it, add both to the space, and afterwards make a 2nd shape with mass > 0 to fix it.